This invention pertains generally to radar seekers for use in guided missiles, and particularly to active seekers operating at frequencies wherein optical techniques may be used to reduce the size, lower the cost, and improve the performance of such seekers.
Anti-armor weapon systems, employing terminally guided submunitions, are being developed to autonomously seek, identify and attack armored targets in a high ground clutter background. In order to provide all weather capability such submunitions will generally employ millimeter-wave radar seekers, and in order to attain the requisite degree of target discrimination the millimeter-wave seeker must employ a relatively sophisticated radar system as, for example, a synthetic aperture radar system or a polarimetric radar system. Either such type of radar is, however, relatively complex. The complexity of such radar system will be appreciated when it is recognized that at an operating frequency of, say, 94 GHz, conventional waveguide dimensions are in the order of 0.050 to 0.100 inches, with tolerances of better than 0.001 inches required in many critical assemblies. Although it may be possible to fabricate such millimeter-wave hardware at somewhat reduced cost using modern robotic techniques, the expense associated with tuning and testing such critically toleranced hardware may well prove to be prohibitive.
The problems of packaging and tuning an active millimeterwave seeker in a conventional submunition will be appreciated when it is recognized that a polarimetric or dual polarization monopulse seeker without a monopulse tracking capability utilizing waveguide components may well require in excess of twenty different waveguide components to control the routing and diplexing of the various signals coming from the transmitter and returning to the receivers. If a monopulse tracking capability were required, then all of the foregoing waveguide components would be required to track each other in both amplitude and phase. At an operating frequency of 94 GHz, each one thousandth of an inch in a waveguide assembly is equivalent to about 2.degree. of phase. It should therefore be appreciated that obtaining the requisite phase and amplitude tracking between the various channels is extremely difficult at best.
Another problem inherent in active millimeter-wave radar seekers utilizing waveguide devices is that of providing sufficient isolation between the transmitter and receiver. This problem is exacerbated by the fact that waveguide switches and circulators providing a high degree of isolation are not generally available at an operating frequency of 94 GHz. Consequently, it is generally required to turn the transmitter off during the interpulse periods of the radar to realize the requisite isolation. This approach, however, requires the use of a complex phase lock control loop, such as that described in copending U.S. application Ser. No. 356,696 (now U.S. Pat. No. 4,470,049 issued Sept. 4, 1984) filed Mar. 3, 1982 and assigned to the same assignee as the present application, to insure that the phase of the transmitter is properly controlled during the pulse transmission periods.
Another problem inherent in millimeter-wave radar systems utilizing waveguide components is that of a relatively low operating bandwidth due primarily to the critical waveguide tolerances. A relatively narrow operating bandwidth increases the susceptibility of the millimeter-wave radar to electronic countermeasures.